Severe thunderstorms moved across Greenwood, South Carolina, late in
the afternoon of Thursday, 28 September 2006. Several reports of wind
damage were received, including this destroyed shed at a business along
the Hwy 72 Bypass on the northwest side of Greenwood. Image courtesy
of Lisa Boggs, Greenwood Index-Journal. Used by permission.

Author's Note: The following report has not been subjected to the scientific peer review process.

1. Introduction

An outbreak of severe thunderstorms happened in the afternoon and evening
of Thursday, 28 September 2006, across parts of north Georgia, the Carolinas,
and Virginia (Fig. 1). In all, the National Weather Service Office at the
Greenville-Spartanburg Airport (GSP) issued 18 Severe Thunderstorm Warnings
on that day, of which 14 were verified with the occurrence of wind damage or
large hail. One severe thunderstorm developed over Elbert County, Georgia,
and first produced damaging wind gusts near Elberton. This severe storm
produced more wind damage as it moved rapidly across Abbeville County, South
Carolina, and eventually evolved into a small mesoscale convective system
across Greenwood and Laurens counties. In particular, wind damage was most
widespread across the area from Elberton, Georgia, to Calhoun Falls, South
Carolina.

(Click here to view a summary of severe weather reports for 28 September
2006.

Figure 1. Wind damage, large hail, and tornado reports for 28 September
2006. Click on image to enlarge.

The outbreak was unusual due to the time of year. Climatologically
speaking, the period from mid-September through late October tends to
have the least occurrence of severe weather across this part of the
United States. Most outbreaks of severe weather during this time of
the year are associated with the passage of tropical cyclone remnants.

2. Synoptic Features and Pre-Storm Environment

The potential for severe weather was recognized early in the morning as
much of the Foothills and Piedmont of the Carolinas was placed in a slight
risk of severe thunderstorms by the Storm Prediction Center (SPC) at
1300 UTC (all times are referenced to Universal Time Coordinated [UTC],
which is Eastern Daylight Time plus four hours). The axis of a deep upper
trough with a strong embedded short wave, located over the Mississippi
Valley at 1200 UTC (Fig. 2), approached the area during the mid and late
afternoon of 28 September. Most notable at 1200 UTC was the band of
strong winds greater than 70 kts diving into the bottom of the trough
over Missouri and Arkansas. The nose of the stronger winds moved around
the bottom of the trough and over the western Carolinas by the time of
the next 500 mb analysis at 0000 UTC 29 September.

By the middle part of the day, a cold front stretched from Louisiana,
across the Tennessee Valley, to the upper Ohio Valley , as shown by the
surface analysis from the Hydrometeorological Prediction Center (HPC) at
1500 UTC (Fig. 3). Although surface moisture ahead of the system was
quite meager, even by early autumn standards (lower to mid 50s dewpoints
on Fig. 4), mid-level cooling associated with the approaching short wave
resulted in mid level lapse rates on the order of 6 degrees Celsius per
kilometer. This combined with the fact that timing was almost optimal
to coincide with maximum heating (most locations east of the mountains
saw full sun through early afternoon) allowed the atmosphere to become
moderately unstable by 1800 UTC, with convective available potential
energy (CAPE) generally in the 1000-1500 J/kg range. In addition, the
highly dynamic nature of the system resulted in unseasonably strong shear
values, with 0-6 km shear of 50-60 kts observed during the event. The
1200 UTC upper air sounding taken at Nashville, Tennessee (KBNA, Fig. 5),
was generally representative of the pre-convective environment expected
over the western Carolinas in the afternoon. The sounding shows the
potential for a CAPE of only 800 J/kg, but strong shear on the order
of 60 kts.

Figure 4. Regional surface plot at 1843 UTC 28 September.
Observations are plotted according to the standard station model.
Click on image to enlarge.

Figure 5. Skew-T log P diagram (upper left) and hodograph (upper right)
for upper air sounding at BNA at 1200 UTC 28 September. The tables at the
bottom summarize several objective parameters used by the SPC to determine
severe weather potential. Click on image to enlarge.

Figure 6. Radar reflectivity on 0.5 degree scan from the KGSP WSR88-D
radar at 1946 UTC. The radar is located at the upper right edge of the
image. Click on image to enlarge.

Although there were several marginally severe discrete cells and line
segments that developed during this event, by far the most interesting
was the discrete cell that produced widespread wind damage from Elberton
to Abbeville, before evolving into more of a linear Mesoscale Convective
System (MCS). This particular thunderstorm had its genesis over northeast
Georgia prior to 2000 UTC. The cell strengthened rapidly as it moved east
from Madison County, Georgia, prompting the issuance of a Severe Thunderstorm
Warning for Elbert County at 2027 UTC. Additional strengthening took place
as the leading edge of the severe thunderstorm bowed outward toward the
city of Elberton at 2041 UTC (Fig. 7). The weaker reflectivity behind the
bowing part of the storm, west of Elberton, is known as a weak echo channel
and is indicative of very strong winds. Based on this scan, a Severe
Thunderstorm Warning was also issued for Abbeville County at 2045 UTC.

Figure 7. As in Fig. 6, except for 2041 UTC. Note the leading edge of
the higher reflectivity bowing outward over Elberton and the relatively
weaker reflectivity immediately behind (west of) the bow west of Elberton.
Click on image to enlarge.

The Elbert County storm developed into a classic "spearhead echo"
(Fujita 1976, Fujita and Byers 1977) as it moved across the area between
Elberton and Calhoun Falls, South Carolina. (For other examples of
spearhead echoes, see these examples from Huntingdon County, Pennsylvania,
and Perry County, Missouri.) These rare, compact, fast-moving cells
(this one was moving at about 47 knots) usually pack quite a wallop, and
this one was certainly no exception. Although no direct measurements of
wind speed were available, numerous reports were received of large limbs
or trees blown down from Elberton to Calhoun Falls.

Figure 8. As in Fig. 6, except for 2054 UTC. Note the "spearhead"
appearance of the higher reflectivity protruding eastward between Elberton
and Calhoun Falls. Click on image to enlarge.

As the Elberton storm moved almost due east into Abbeville County, the
path of storm became increasingly perpendicular to the beam from the
KGSP radar. As a result, the Doppler velocity was most likely sampling
a smaller and smaller component of the true wind velocity in the storm.
However, the WSR-88D radar at the NWS office in Columbia (KCAE) was in a
more favorable location to view the rear-to-front wind velocity in the
spearhead echo, as the storm was moving toward that radar (Fig. 9).
The velocity along the radial from the KCAE radar was greater than 60 kts
at the level of the radar beam, which at that location was approximately
2.4 km above ground level (Fig. 10).

Figure 9. Radar reflectivity on 0.5 degree scan from the KCAE WSR88-D
at 2056 UTC, showing the spearhead echo moving into Abbeville County.
The radar is located off the right side of the image. Click on image
to enlarge.

Figure 10. As in Fig. 9, except for radial velocity. Compare the
location of the higher Doppler velocities with the location of the
spearhead echo in Figure 9. Click on image to enlarge.

The Elberton/Abbeville storm eventually merged with other cells to produce
more of an elongated (although still quite compact), "typical" MCS over
the Lower Piedmont of South Carolina. Additional Severe Thunderstorm
Warnings were issued for Greenwood County at 2120 UTC and for Laurens
County at 2122 UTC in advance of the MCS. Even though the "spearhead
echo" was no longer apparent, the MCS continued to produce fairly
widespread tree damage across Greenwood County and the southern half of
Laurens County.

4. Discussion

Although meteorologists often look for mid-level dry air when attempting
to assess the potential for damaging winds, either due to initiation
of negatively bouyant air or enhancement of pre-existing downdrafts
through entrainment of dry air, it is important to remember that organized
convection can develop intense subsiding air currents through various
pressure gradient forcings. In the 28 September case, there wasn't much
mid-level dry air, but organized convection was a given, as a consequence
of the very strong shear. Subsiding air streams generated by the Elberton
storm were probably enhanced by the dry air in the sub-cloud layer.
Surface dewpoint depressions of ~30 deg. F were observed in the pre-
convective environment across the southern Upstate. Combine these factors
with the fact that the cell in question was moving at 47 kts, and you have
a serious wind damage threat on your hands.

The Elberton/Abbeville storm was a good example of how storm history plays
a role in the warning process when it comes to organized convection.
Systems like these produce deep, nasty cold pools that are being continuously
reinforced, and they don't easily give up the ghost. Often, when the lead-
line convection appears to be weakening, it probably just means that the
cold pool is becoming too deep and strong to force air parcels to their
level of free convection. In these cases, wind damage is still quite
possible, even likely due to the strength of the cold pool.

5. Summary

In the late afternoon of Thursday, 28 September 2006, an organized severe
thunderstorm produced a swath of wind damage across the area roughly
bounded by Elberton, Georgia, and Iva, South Carolina, across much of
Abbeville County and Greenwood County, to the southern half of Laurens
County, South Carolina. The most widespread wind damage occurred in the
area between Elberton and Calhoun Falls, coincident with a spearhead echo
observed on the KGSP radar. All reports of wind damage were preceded by
a Severe Thunderstorm Warning. For the entire event, the NWS office in
Greer, South Carolina, issued 18 Severe Thunderstorm Warnings. Of those
warnings, 14 were verified by reports of wind damage or large hail. There
was one missed event. Verification scores are as follows:
Probability of Detection: 94%
False Alarm Ratio: 22%
Critical Success Index: 0.74
Average Lead Time: 8.7 minutes

Tree damage around Calhoun Falls, South Carolina, courtesy of
Darlene Cox. Click on images to enlarge.

Additional images of tree damage near Greenwood, South Carolina, courtesy
of Lisa Boggs, Greenwood Index-Journal. Click on images to enlarge.

Acknowledgements

Chris Horne contributed additional insight as to the warning process
during the event. The plot of storm reports in Figure 1, upper air
charts, and the upper air sounding in Figure 5 were obtained from the
Storm Prediction Center. Surface analyses were obtained from the
Hydrometeorological Prediction Center. The regional surface plots and
satellite imagery were obtained from the University Corporation for
Atmospheric Research. All radar imagery graphics were created using the
Java NEXRAD viewer obtained from the National Climatic Data Center. Our
gratitude is given to Lisa Boggs of the Greenwood Index-Journal and to
Darlene Cox for the use of their storm damage images.